The number of different wireless communication services and systems has increased during last years, and intensive development for new services is continuing. Several different cellular networks are in use. Examples of different cellular networks include GSM (Global System for Mobile communication), PCS (Personal Communications Services) and 3rd generation mobile communication networks. These networks may use different frequency bands in different parts of the world. In addition to cellular telecommunications, other wireless services have been developed. Examples of such wireless services include digital broadband broadcasting, such as DVB-T (Digital Video Broadcasting-Terrestrial) and DVB-H (Digital Video Broadcasting-Handheld) providing digital television transmission and reception.
Many modern terminals are already configured to support more than one wireless service. For example, terminals operating in time division multiple access systems, such as GSM, are also capable of receiving digital broadband broadcast transmissions, such as DVB-H transmissions.
One of the problems in designing terminals supporting more than one wireless service is that the frequency bands supported by the services may be close to each other or, in some cases, even overlap. Thus, when the user of the terminal is using a first wireless service, communication using a second service may cause interference to the first service.
For example in the USA, a channel for a DVB-H service is allocated at frequency band of 1670 to 1675 MHz. In Europe, a frequency band allocation for the DVB-T and DVB-H service extends from 470 to 862 MHz in the ultrahigh frequency (UHF) band. It is also possible that future implementations in Europe and in the USA may utilize frequencies in higher or lower UHF frequencies as well. The frequency allocations are problematic since the cellular operation may cause in the terminal strong interference to the DVB-H reception, for example, if both of these services are operated simultaneously. For example, wideband noise of a transmitter operating in a GSM 900 or Extended GSM (EGSM) system (the transmission frequency range in these systems extends from 880 MHz to 890 MHz (EGSM) or from 890 MHz to 915 MHz (GSM 900)) desensitizes the uppermost DVB-T/H reception channels in Europe and wideband noise of PCS band transmission (1850 to 1990 MHz) desensitizes the DVB-H reception in the USA.
The interference problem is especially evident in terminals supporting both digital broadband broadcast reception and time division multiple access cellular services. The normal operation of a cellular transceiver may cause interference to the digital broadband broadcasting reception. More closely this means that the cellular transceiver typically transmits broadband noise in addition to the wanted signal. The broadband noise couples via a cellular antenna to a digital broadband broadcast reception antenna, folds on top of the digital broadband broadcast reception frequencies, and disturbs or even prevents reception. Another problem is that even the wanted cellular transmission signal can produce a blocking effect in digital broadband broadcast reception if the cellular transmission band is very close to the digital broadband broadcast transmission band making the transition band between the digital broadband broadcasting and cellular systems very short.
Broadband noise is typically produced by a power amplifier in the cellular receiver. In many cases, the broadband noise produced, for example, by the GSM transceiver is in-band interference, for example, for a DVB-H receiver and cannot be anymore filtered in the DVB-H receiver. The broadband noise has been suggested to be filtered in the cellular transceiver. In accordance with current understanding, however, it is not seen practically possible to make a filter which would be steep enough with low enough loss for cellular operation. Similarly, concerning the wanted cellular transmission signal near the digital broadband broadcast reception band, making a steep enough input filter which would filter the wanted cellular transmission signal in the digital broadband broadcast receiver is considered problematic or even impossible. According to another suggestion to reduce interference the use of block periods has been proposed. The intention was to block cellular transmission during digital broadband broadcast reception in order to avoid interference. However, the use of block periods in the conventional cellular systems would typically lead into irrecoverable damages in the quality of the cellular signal in certain services, such as voice calls.